The disclosures of the Japanese Patent Applications Nos. Hei-8-235832 filed on the Aug. 20, 1996 and Hei-9-31441 filed on the Jan. 31, 1997 are incorporated herein by reference.
The present invention relates to a method and an apparatus for purifying a gas containing contaminants. More specifically, the present invention relates to a method and an apparatus for purifying a gas by producing microparticles of the contaminants present in a gas and decomposing the resultant microparticles of contaminants with a photocatalyst for facilitating the removal thereof.
It was considered satisfactory in semiconductor industries in the past to remove only solid particles such as dust from a gas such as air in a clean room. Methods for removing solid particles can be classified broadly into 2 categories: (1) mechanical filtration methods (e.g. HEPA (High Efficiency Particulate Air) filter); and (2) methods for trapping microparticles electrostatically (e.g. MESA filter). Methods included in the category (2) comprise charging microparticles electrically with a high electrical voltage and filtering the charged microparticles with an electrically conductive filter. Gaseous contaminants, however, cannot be removed by any method of either category.
Development of semiconductors of higher quality and finer precision has made it necessary to remove not only dust-like solid particles but also gaseous contaminants. Gaseous contaminants include: organic compounds including phthalic esters; organosilicon compounds including siloxane; acidic gases including sulfur oxides (SOx), nitrogen oxides (NOx), hydrogen chloride (HCl) and hydrogen fluoride (HF); as well as basic gases including NH3 and amines. Amines may be included among organic compounds also. Anions such as NO3xe2x88x92, NO2xe2x88x92, SO42xe2x88x92, etc. have characteristics and exert adverse effects similar to acidic gases, and therefor, are considered as a member of acidic gases out of convenience. Likewise, cations such as NH4+, etc. have characteristics and exert adverse effects similar to basic gases, and therefor, are considered as a member of basic gases for convenience.
Organic compounds or organosilicon compounds, when deposited onto the surface of a wafer (substrate), may have a negative effect on the affinity (drapability) of a substrate for a resist. Decreased affinity may exert a harmful influence on both the film thickness of a resist and the adhesion of a substrate to a resist (xe2x80x9cAir Cleaningxe2x80x9d, Vol. 33, No. 1, pp. 16-21, 1995). For example, SOx may bring about defective insulation in an oxide layer. NH3 may produce ammonium salts that are responsible for the blooming (poor resolution) of a wafer (Realize Inc., xe2x80x9cSaishin Gijyutsu Kozau, Shiryo-shuxe2x80x9d, Oct. 29, 1996, pp. 15-25, 1996). For the aforementioned reasons, such gaseous contaminants may diminish the productivity (yield) of semiconductor products.
It was also considered satisfactory in the past to remove gaseous contaminants to a level of ppm. It has become required now to remove gaseous contaminants to a level of ppb. Among organic compounds, alkanes such as methane and the like are not so reactive as to exert an unfavorable influence on a semiconductor, and hence are not required to be removed to a level of ppb.
Removal of contaminants including organic compounds, especially gaseous organic compounds is described below in more detail.
Known methods for removing organic compounds include decomposition by combustion, catalytic decomposition, removal by adsorption, decomposition with O3 and the like. These known methods, however, are not effective in removing organic compounds present in low concentrations in air for feeding a clean room.
In a clean room, contamination with organic compounds of an extremely slight concentration cannot be ignored. External organic compounds may be introduced into a clean room. For example, outdoor air is contaminated with organic compounds originating from exhaust gas of cars or those resulting from degassing of polymer products. On the other hand, internal organic compounds may be generated in a clean room. For example, polymer materials (e.g. polymeric plasticizers, releasers, antioxidants and the like) which are used for constructing a clean room are producers of organic gases (xe2x80x9cAir Cleaningxe2x80x9d, Vol. 33, No. 1, pp. 16-21, 1995). Synthetic polymers are used in packing materials, sealants, adhesives and wall-forming materials in a clean room. In addition, plastic containers are disposed in a clean room. These synthetic polymers may evolve a trace amount of organic gases. More particularly, sealants and the production units thereof may give off gaseous siloxane, and plastic containers may give off gaseous phthalic esters. It has recently been found that gas evolves also from polymer materials employed in a production unit. A process unit is partially or entirely surrounded by plastic plates which also produce organic gas. A variety of solvents (e.g. alcohols, ketones, etc., which are necessary for operations in a clean room are also a contamination source.
As stated above, a clean room is contaminated variously and heavily with not only organic compounds attributable to external air but also with organic compounds and organosilicon compounds that are generated internally.
In view of energy saving considerations, recycling of air in a clean room has become more frequent recently. In consequence, organic gases are progressively concentrated in a clean room, leading to heavier contamination of the base materials of a wafer and a substrate. These organic compounds are likely to deposit onto the bodies (e.g., starting materials and semi-fabricated products of a semiconductor wafer, a glass substrate, etc.) placed in a clean room, adversely affecting them.
A contact angle indicates a degree of contamination on a wafer substrate with organic compounds and organosilicon compounds. The contact angle refers to the angle formed by the water and the surface of a substrate when the surface is wet with water. The surface of a substrate, when covered with a hydrophobic (oily) substance, becomes more water-repellent and less wettable, hence the contact angle of water on the surface of a substrate becomes larger. In other words, when the contact angle is larger, the degree of contamination is higher. On the contrary, when the contact angle is smaller, the degree of contamination is lower.
When a substrate is contaminated with organic compounds and organosilicon compounds, its affinity (drapability) for a resist decreases, imparting an unfavorable influence on the resist and the film thickness or on the adhesion of the substrate to the resist, that may result in lower quality and a lower yield.
Techniques in the high-technology field have made remarkable progress in realizing semiconductor devices of a maximal precision and a minimal size. In consequence, it has become necessary for a clean room to be free from organic compounds normally present in the air of the level that had conventionally been able to be ignored (an extremely low concentration of the ppb level) [Preparatory Manuscripts for the 39th Meeting of the Applied Physical Society, p.86 (1992, Spring); xe2x80x9cAir Cleaningxe2x80x9d, Vol. 33, No. 1, pp. 16-21, (1995)], as well as gaseous contaminants including SO2, HF, NH3 [xe2x80x9cUltra Clean Technologyxe2x80x9d, Vol. 6, pp. 29-35 (1994)]. Because, it has been revealed that the presence of these gaseous contaminants diminished remarkably the productivity (yield). The present invention is aiming to efficiently remove these gaseous contaminants.
The present inventors have proposed a method for removing hydrocarbons present in a gas comprising the steps of: irradiating the gas with an ultraviolet ray and/or a radiation ray so as to produce microparticles from the hydrocarbon; and trapping the resultant hydrocarbon microparticles with a filter or charging the hydrocarbon microparticles electrically with a photoelectron and trapping the resultant charged microparticles (Laid Open Japanese Patent Application No. Hei-5-96125). A similar method can be applied also to noxious matter present in a gas (Laid Open Japanese Patent Application No. Hei-4-243517).
Using the methods mentioned above, however, trapped microparticles become accumulated on the filter or in the part for trapping the charged microparticles, thus requiring frequent changing of the filter or the trapping part. Further, when the accumulated microparticles fall from the filter or from the trapping part, the fallen microparticles, even if they are in extremely small amounts, inadvertently contaminate a gas to be purified. Therefore, it is considered preferable to decompose contaminants than to remove them.
A conventional removing method is now described with reference to FIGS. 16 and 17. As shown in FIG. 16, the air which is fed to a clean room 1 in a recycled manner is composed of the external air that is fed via a pipe 2 and is cleared of coarse particles through a prefilter 3 and the internal air that is drawn out of the clean room 1 through an air outlet 4. Both airs are combined in a fan 5, conditioned in temperature and moisture with an air conditioner 6 and cleared of microparticles with a HEPA filter 7. The air in the clean room is kept at a purity (class) of the order of 10,000. In this specification, the term xe2x80x9cclassxe2x80x9d refers to the number of particles having a particle diameter of not less than 0.1 xcexcm that are present per cubic feet.
A clean bench 51 is disposed in the clean room 1 to trap and remove a trace amount of hydrocarbons and microparticles (particulate matter).
Organic compounds present in the clean room 1 may consist presumably of those that originating in external air introduced through the pipe 2 (those that are presumably discharged from cars and synthetic resins) and those that are produced during operations in the clean room.
The clean bench 51 comprises mainly a microparticle-producing section 48, a microparticle-charging section 49 and a section for trapping charged microparticles 50. A highly pure air (of class 10) that is freed of both organic compounds and coexistent microparticles is fed above a working table 53, where operations are being carried out.
In other words, air having a purity (class) in the order of 10,000 and containing a trace amount of organic compounds originating in the clean room 1 is directed with a fan (not shown) toward the clean bench 51. At the clean bench 51, the microparticle-producing section 48 is provided for irradiating the air with an ultraviolet radiation of a short wavelength so as to produce microparticles of organic compounds contained in the air. Then, in the microparticle-charging section 49, the microparticles are electrically charged efficiently with photoelectrons emitted by a photoelectron-emitting material as described hereinbelow. The resultant charged microparticles are trapped and removed in the section for trapping charged microparticles 50 that follows. In this manner, air above the working table 53 can be maintained to be highly pure and free of organic compounds.
A movable shutter is provided on the clean bench 51 for facilitating introduction and/or withdrawal of instruments and products into and/or out of the working table 53.
FIG. 17 shows schematically a microparticle-producing section 48, a microparticle-charging section 49 and a section for trapping charged microparticles 50. These sections are described just below with reference to FIG. 17.
In other words, air 54 aspirated through a fan (not shown) and containing a trace amount of organic compounds is filtered through a prefilter (not shown), and then irradiated with an ultraviolet radiation of a short wavelength in the microparticle-producing section 48 that is mainly consisting of an UV lamp 55. Organic compounds present in the air 54 are transformed into microparticles 56 by UV irradiation. These microparticles 56, together with naturally-occurring microparticles 57 already present in the introduced air 54, are electrically charged in the microparticle-charging section 49 so as to become charged microparticles 58.
The microparticle-charging section 49 is mainly composed of an UV lamp 59, a photoelectron-emitting material 60 (herein consisting of a glass material having a surface coated with an Au thin layer of a thickness of 5 to 50 nm, for example) and an electrode material 61 for generating an electrical field. The photoelectron-emitting material 60 is irradiated with the UV lamp 59 in the presence of an electrical field so as to emit photoelectrons 62, which in turn supply the microparticles 56, 57 with an electrical charge so as to produce the charged microparticles 58, which can then be trapped in the section for trapping charged microparticles 50 that follows. The section 50 consists of a material for trapping the charged microparticles. Reference numeral 63 indicates an UV-transmissive material. Reference numeral 64 indicates a highly pure air that is dust-free and free from organic compounds.
The arrangement as stated above is suffered from the problems as set forth below:
(1) Microparticles that were produced from organic compounds upon irradiation with an ultraviolet ray and/or a radiation ray often failed to result in complete trapping with a filter or complete charging and trapping with photoelectrons, depending on the irradiation conditions and the kinds of organic compounds. This is probably because some kinds of organic compounds tend to produce microparticles of an extremely small size. Or else, the chemical composition of organic compounds may be responsible for it. In case when the trapping efficiency was low, a trapping section having a larger volume was required, hence making the whole apparatus larger.
(2) Produced particulate matter can be trapped at the trapping section 50. Consequently, this particulate matter tends to accumulate in the trapping section during a long-term continuous operation. This requires a design of a trapping section 50 having a higher trapping volume, thus making the size of an apparatus larger.
On the other hand, the present inventors have proposed the use of a photocatalyst in a system for removing organic compounds (Japanese Patent Applications Nos. Hei-8-31230 and Hei-8-31231). In this system, however, organic compounds in a low concentration are decomposed with a photocatalyst so slowly that the decomposition thereof is very time-consuming. In other words, diethylhexyl phthalate (DOP) and siloxane present in the natural air and the air in a clean room are only in a concentration as low as about 1 ppb each.
Further, photocatalysts cannot effectively remove acidic gases such as SO2, NO, HCl and HF. In particular, sulfur-containing compounds such as sulfur oxides, hydrogen sulfide, thiophene and thiols, when present at a high concentration, may sometimes act as a catalytic poison to the photocatalysts. Even if these compounds can avoid acting as a catalytic poison, they might adversely influence on the photocatalysts after a long-term operation.
The present invention is directed to solve the problems as set forth above.
According to one aspect of the present invention, there is provided a method for purifying a gas containing a contaminant comprising a microparticle-producing step for irradiating the gas with an ultraviolet ray and/or a radiation ray so as to produce microparticles of the contaminant, a contact step for contacting the microparticles of the contaminant with a photocatalyst and a first decomposition step for irradiating the photocatalyst with a light so as to decompose the contaminant being in contact with the photocatalyst. Organic compounds (except for alkanes), organosilicon compounds and basic gases can be oxidatively decomposed with a photocatalyst. Even when contaminants are present in small amounts, they can be concentrated locally by transforming into microparticles, and hence can be oxidatively decomposed with a photocatalyst efficiently.
In the microparticle-producing step, an ultraviolet ray and/or a radiation ray having a wavelength of not more than 260 nm is preferably used. Contaminants can aggregate through a radical reaction to produce microparticles.
Preferably, the gas contains water or gaseous oxygen in a concentration of not less than 1 ppb. More preferably, the gas contains water or gaseous oxygen in a concentration of not less than 100 ppb. It is believed that water or gaseous oxygen acts on the surface of a photocatalyst by supplying it with OH radical to induce activation of the photocatalyst. The OH radical probably acts as an oxidant in the presence of the photocatalyst.
It is preferred that: a gas contains gaseous oxygen of at least 1 ppm; the gaseous oxygen present in the gas is transformed into ozone at the microparticle-producing step; and the method further comprises a second decomposition step for decomposing the resultant ozone.
More preferably, the method comprises a removal step for removing contaminants. Preferably, the contaminants contain acidic or basic compounds, and more preferably, the contaminants contain at least one species selected from the group consisting of nitrogen oxides (NOx), nitrogen oxide ions, sulfur oxides (SOx), sulfur oxide ions, hydrogen sulfide, hydrogen fluoride, ammonia and amines.
The removal step may precede the microparticle-producing step. Alternatively, the removal step may follow the microparticle-producing step and precede the first decomposition step. The latter order is suitable when there is contained any contaminant serving as poison to a photocatalyst. More precisely, when sulfur-containing compounds such as sulfur oxides, hydrogen sulfide, thiophene and thiols are present, it is preferred that these compounds are removed prior to the treatment with the photocatalyst. Alternatively, the removal step may follow immediately after the first decomposition step.
Preferably, the removal step is carried out by means of one or more of a filter, an adsorbent, an ion exchanger and a photoelectron. Photoelectrons can supply contaminants with an electrical charge so as to facilitate the trapping of the contaminants.
Preferably, the photocatalyst is composed of a matrix and a catalytically active component which is carried on the matrix and which is preferably in the form a particle. More preferably, the matrix is in the form of a honeycomb structure provided with partitions defining at least 2 through-holes, a bar body or a wall member, and the catalytically active component is semiconductor.
According to the second aspect of the present invention, there is provided an apparatus for purifying a gas containing a contaminant comprising: a microparticle-producing section having a source of an ultraviolet ray and/or a radiation ray; and a decomposition section having a photocatalyst and a light source for irradiating the photocatalyst, the decomposition section being connected to the microparticle-producing section.
Preferably, the apparatus is provided with a gas inlet and a gas outlet and is disposed in a manner that the gas inlet, the microparticle-producing section, the decomposition section and the gas outlet are arranged successively downstream.
More preferably, an ozone-decomposing material is provided downstream to the microparticle-producing section.
In addition, it is preferred to provide a removal section for removing acidic and/or basic compounds. Preferably, the removal section comprises one or more means selected from a filter, an adsorbent and an ion exchanger as well as a photoelectron-emitting means and a means for trapping charged contaminants.